The problem with reading the line about deceit as a reference to Florida is that Ferris never accuses her of deceitfulness or even seems particularly angry with her. To the extent that his jealousy makes him angry, that anger seems to be directed at Don Ippolito - or perhaps more at himself for introducing Don Ippolito to the Vervains and thereby causing this whole tangle.

I'm not sure why he clings so tenaciously to this jealousy after Don Ippolito explains, though. Possibly he thinks Florida does love Don Ippolito, but turned him down because she's so intensely religious that she couldn't bear for a priest to break his vows for her?

But this is just conjecture.. The book doesn't explain it; perhaps it can't explain it, because at bottom it's an unreasonable feeling.